1. Field of the Invention
The invention relates in general to a circuit substrate, and more particularly to a circuit substrate using a Laser cutting and drilling technique to form a built-in inductor element.
2. Description of the Related Art
Referring to FIG. 1A and FIG. 1B, a top view of a conventional spiral inductance and a side sectional view of a conventional spiral inductance are shown. Conventional spiral inductor 10, which is disposed on the surface of a substrate 14, is electrically connected with a grounding layer 16 through a micro-via 12. Magnetic line of force 18 is perpendicular to the grounding layer 16 and is reflected by the grounding layer 16 because the grounding layer 16 produces shielding effect to change the distribution of electrical and magnetic fields. However, the space between the spiral inductor 10 and the grounding layer 16 cannot be used. Given that the inductance is proportional to the square of the number of coils of the inductance and that the plane disposition of the spiral inductor 10 is restricted by limited layout space, so both the inductance value and the quality factor Q cannot be further improved effectively.
In the design of a conventional spiral inductor 10, normally, a maximum quality factor under required frequency of operation is adopted, so as to provide an inductance value required for the usable region of the substrate. To reduce the eddy current loss on the substrate and reduce the amount of capacitance coupled to the substrate, a conventional spiral inductor had better to be kept away from the substrate of a semiconductor as far as possible. Therefore, the space between the conventional spiral inductor 10 and the grounding layer 16 (the reference plane) must have a certain height H and cannot be over-compressed, which contradicts with the trend of miniaturization. On the other hand, the grounding layer cannot be left out lest electromagnetism might be coupled to and interfere with adjacent elements. Moreover, a larger micro-via 12 and spiral inductor 10 will occupy more layout space of the substrate surface.
It is worthy mention that normally the quality of an inductor is determined according to the inductance value and the quality factor Q. The value of Q, which is an important characteristic of the inductance, is used to measure the purity of an inductance. For an inductance, a higher Q value means a lower energy loss.
A simple structured LRC circuit 20 is used for a detailed illustration. As shown in FIG. 2, an inductance 22, a resistance 24 and a capacitance 26 are serially connected together. In the circuit, oscillation occurs because the stored energy moves between the inductance 22 and the capacitance 26. Meanwhile, the quality factor Q measures the decay of the oscillation: the higher the Q value, the smaller the decay. The relation between the Q value and the inductance value is expressed as Q=ωL/R. In other words, the higher the inductance value L, the larger the Q value.